In power generating plants, which are equipped with a steam turbine plant for power generation, it can be necessary, depending upon the current power demand, to shut down an individual steam turbine or a plurality of steam turbines and, depending upon requirement, to re-engage them. A quick starting of the respective steam turbine plant in this case is of vital importance. This especially applies to longer shutdown periods, especially after a cold start and after a hot start, for example after a weekend shutdown. According to the prior art, during the starting process a steam generator is first of all run up or heated in order to increase the steam temperature and the steam pressure. As soon as a predetermined starting temperature and a predetermined starting pressure and also a predetermined starting quality for the steam are stable, a starting process for starting the steam turbine is commenced. For this purpose, inter alia, live steam valves are opened to a greater or lesser degree. In this case, the values for the starting temperature, for the starting pressure and for the starting quality of the steam are selected so that after starting the steam turbine a no-load operation or an on-load operation with low load can be realized for the steam turbine. The steam in this case has a conductivity, the value of which has to lie within a predetermined range in order to avoid damage to the steam turbine as a result of impure steam.
During the starting of a steam turbine plant, therefore, the conductivity of the steam is continually determined and only when the steam has fallen below a certain limiting value is it allowed to flow into the steam turbine.
The values for the starting temperature, for the starting pressure and for the starting quality of the steam are selected so that after starting the steam turbine a no-load operation or an on-load operation with low load can be realized for the steam turbine. These parameters must be stable by the time of the commencement of the actual starting process. Depending upon the type of power plant and type of construction of the boiler, or size of the power plant, about 1 to 3 hours can regularly elapse during this. As a result of the exposure to superheat steam admission, high material stresses, due to the thermal expansion stresses which occur, are regularly brought about during starting from a cold machine state. Monitoring by a measurement technique of the thermal expansion stresses is typically carried out today. In this case, an increased interest exists in shortening the starting times for such a steam turbine plant in order to thereby meet the economical efficiency of the steam turbine plant, or of a power generating plant which is equipped with it.
The starting process is customarily commenced only when a predetermined starting quality exists for the steam, especially with regard to cleanliness and pH value. Also, the prewarming process is preferably commenced only when the steam has a predetermined prewarming quality, wherein the starting quality is higher than the prewarming quality. The cost for achieving a high steam quality is relatively high.
A steam turbine which can comprise a plurality of turbine sections is understood by a steam turbine in the sense of the present application. The turbine sections in this case can be designed for different steam parameters, such as temperature and pressure. In this case, high-pressure, intermediate-pressure and low-pressure turbine sections are known. Superheated steam, which can have a temperature of up to 620° C., as a rule flows into the high-pressure turbine section. Furthermore, this superheated steam can have a pressure of up to 300 bar. The superheated steam is also referred to as hot steam. If saturated steam is separated from a solid or condensate and is heated at constant pressure, then the steam becomes increasingly unsaturated. This steam is referred to as hot steam or superheated steam.
The steam space above bottoms or condensate has absorbed the largest amount of molecules possible for an existing steady-state temperature: this steam is referred to as wet steam, dry steam or saturated steam.
An intermediate-pressure turbine section, however, is formed in such a way that the expanded steam from a high-pressure turbine section reaches a reheater, wherein the temperature of the steam is increased in the reheater, and then flows into the intermediate-pressure turbine section. The temperature of the steam which flows into the intermediate-pressure turbine section in this case is at about 600° C., and has a temperature of about 80 bar. The steam which issues from the intermediate-pressure turbine section is finally directed to a low-pressure turbine section.
The subdivision into high-pressure, intermediate-pressure, and low-pressure turbine sections is not uniformly applied within the specialist field. Therefore, the steam parameters, such as temperature and pressure, cannot be used as a single differentiating criterion between a high-pressure, intermediate-pressure and low-pressure turbine section.
Methods for operating steam turbines with reheating of the steam which issues from the high-pressure turbine section and flows into an intermediate-pressure turbine section, are known. As a result of the reheating, the temperature of the steam, which has already performed work in the high-pressure turbine section of a steam turbine, is increased again and therefore increases the available gradients before the steam reaches the low-pressure section of the turbine. Consequently, the efficiency of the plant is increased.
A further advantage of the operation of steam plants or combined cycle power plants with reheating of the steam is that as a result of the reheating, the exhaust wetness of the steam in the final stages of the turbine is reduced and consequently the fluidic quality and the service life is improved.
Reheating is used in steam turbines when the steam becomes too wet during expansion in the machine. The steam, after flowing through a number of stages, is then directed out of the turbine to the reheater, and after that, resupplied to the turbine. In the case of very high pressure gradients, a multiple reheating is used so as not to retain too much steam wetness in the final stage.